Postfix has become the preferred mail transfer agent for Linux administrators, replacing Sendmail with a more secure, modular, and easier-to-configure architecture. Properly configured, Postfix can handle thousands of messages per hour while maintaining strong defenses against spam and abuse.
Anti-Spam and Relay Prevention
Configure Postfix to reject mail from unknown senders during the SMTP conversation using smtpd_sender_restrictions and smtpd_recipient_restrictions. Enable the reject_unknown_sender_domain and reject_unauth_destination checks to prevent open relay abuse. Implement SMTP-time rejection rather than accepting and bouncing messages, as bouncing generates backscatter spam.
Integrate SpamAssassin or Amavisd-new as a content filter to score incoming messages and reject or quarantine likely spam. Use DNS-based blocklists (RBLs) like Spamhaus and SORBS to reject connections from known spam sources during the SMTP handshake, reducing the processing load on content filtering. Greylisting provides an additional layer of defense by temporarily rejecting first-time sender/recipient pairs, which stops most spam bots that do not retry.
Implement SPF, DKIM, and DMARC for your outbound mail to improve deliverability and prevent domain spoofing. SPF publishes authorized sending servers in DNS, DKIM adds cryptographic signatures to outgoing messages, and DMARC defines how receivers should handle messages that fail SPF or DKIM checks. Together, these technologies protect your domain's reputation and help ensure your legitimate mail reaches its destination.